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-acetylation of ribosomal protein S18Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461, USA
(RECEIVED April 17, 2008; FINAL REVISION June 20, 2008; ACCEPTED June 20, 2008)
The three ribosomal proteins L7, S5, and S18 are included in the rare subset of prokaryotic proteins that are known to be N
-acetylated. The GCN5-related N-acetyltransferase (GNAT) protein RimI, responsible for the N
-acetylation of the ribosomal protein S18, was cloned from Salmonella typhimurium LT2 (RimIST), overexpressed, and purified to homogeneity. Steady-state kinetic parameters for RimIST were determined for AcCoA and a peptide substrate consisting of the first six amino acids of the target protein S18. The crystal structure of RimIST was determined in complex with CoA, AcCoA, and a CoA-S-acetyl-ARYFRR bisubstrate inhibitor. The structures are consistent with a direct nucleophilic addition–elimination mechanism with Glu103 and Tyr115 acting as the catalytic base and acid, respectively. The RimIST-bisubstrate complex suggests that several residues change conformation upon interacting with the N terminus of S18, including Glu103, the proposed active site base, facilitating proton exchange and catalysis.
Keywords: protein N
-acetylation; bisubstrate inhibition; GNAT structure; ribosomal protein
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